North Korea’s Alarmed Neighbors Consider Deploying Deadlier Weapons

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Japanese tanks during a 2016 exercise in Gotemba, southwest of Tokyo. Japan’s government said on Tuesday that North Korea may have already acquired nuclear warheads.CreditCreditEugene Hoshiko/Associated Press

TOKYO — North Korea’s rapidly advancing nuclear program has prompted politicians in Japan and South Korea to push for the deployment of more powerful weapons, in what could lead to a regional arms race.

Some of the new capabilities under consideration in Tokyo and Seoul, Washington’s closest Asian allies, are politically contentious. Adopting them would break with decades of precedent and could require delicate diplomatic finessing. Other military options are already being rolled out or will be soon.

President Trump also suggested on Tuesday that military options were at least under consideration. Using chilling language that evoked the horror of a nuclear exchange, Mr. Trump threatened to unleash “fire and fury” against North Korea if it endangers the United States.

Undaunted, North Korea responded several hours later, saying that it was considering a strike that would create “an enveloping fire” around Guam, the western Pacific island where the United States operates a critical Air Force base.

In a military policy review published on Tuesday, the Japanese government focused on the threat from North Korea, whose leader, Kim Jong-un, has ordered more than a dozen missile tests this year. Some of those missiles have splashedinto waters close to Japan.

“North Korea’s development of ballistic missiles and its nuclear program are becoming increasingly real and imminent problems for the Asia-Pacific region including Japan, as well as the rest of the world,” the government in Tokyo said in its annual defense white paper. “It is possible that North Korea has already achieved the miniaturization of nuclear weapons and has acquired nuclear warheads.”

That bleak assessment is likely to feed a growing debate in Japan about whether the country should acquire the means to launch pre-emptive military strikes — attacks that could destroy North Korean missiles on the ground before they are fired at Japan or other targets. Lawmakers are already pushing for such capacities; acquiring them would amount to a profound change for Japan, whose post-World War II Constitution renounces war.

Japan has long limited its military to a strictly defensive role. Although successive governments have argued that, in theory, striking an enemy pre-emptively to thwart an imminent attack would be an act of self-defense, and therefore constitutional, the country has mostly avoided acquiring the kind of armaments it would need to do so. They include long-range cruise missiles, air-to-ground missiles and refueling aircraft that extend the range of fighter jets.

Some senior officials are now arguing that Japan should acquire such weapons.

“North Korea’s missile launches have escalated tensions, both in terms of quality and quantity,” Itsunori Onodera, Japan’s new defense minister, said on Friday, a day after Prime Minister Shinzo Abe installed him in the post in a cabinet reshuffle. “I would like to study if our current missile defense is sufficient.”

In March, Mr. Onodera led a committee of lawmakers from the governing Liberal Democratic Party in recommending that Japan consider acquiring the ability to carry out pre-emptive strikes. His views could be reflected in an updated five-year military strategy that is to be published by the Defense Ministry next year.

Japan has already committed to buying advanced F-35 fighter planes, and it is shopping for an upgraded land-based missile defense system to improve its chances of shooting down any incoming North Korean missiles.

North Korea escalated its standoff with the United States and other nations on Tuesday, warning that it would take unspecified “physical action” in retaliation for new sanctions the United Nations Security Council adopted over the weekend.

Officials and analysts say they still doubt that North Korea has mastered all of the technologies needed to deliver a nuclear payload on an intercontinental ballistic missile. But the country’s latest ICBM test, on July 28, was nonetheless alarming, showing that its missiles now have a potential range that could extend to much of the continental United States.

Even more than Japan, South Korea is working to build its monitoring and striking abilities, including with radar and remote-controlled reconnaissance planes to track and neutralize North Korean missiles in pre-emptive attacks.

Some want to go further. On Monday, South Korea’s conservative opposition group, the Liberty Korea Party, issued a statement favoring the deployment of American tactical nuclear weapons in the country, the Yonhap news agency reported.

“Peace will come when we achieve a balance of power, not when we are begging for it,” the party’s leader, Hong Joon-pyo, was quoted as saying.

After the North’s ICBM tests, South Korea’s new president, Moon Jae-in, reversed his decision to suspend the deployment of an advanced American missile defense system. He also asked the United States to let the South build more powerful ballistic missiles, a move that would require Washington’s approval under the terms of a bilateral treaty.

Some opinion surveys have indicated that most South Koreans favor their country’s developing nuclear weapons of its own, to counter the North’s, though Mr. Moon opposes the idea.

Hideshi Takesada, a specialist on defense issues at the Institute of World Studies at Takushoku University in Tokyo, said that if South Korea acquired nuclear weapons, Japan might rethink its longstanding aversion to them — despite its traumatic experiences at the end of World War II, when American atomic bombs devastated the cities of Hiroshima and Nagasaki.

“If South Korea went nuclear, that debate would happen in Japan, too,” Professor Takesada said.

In a carefully hedged answer, the prime minister said, “At the present time, we are not planning any specific deliberations about possessing” weapons for a pre-emptive strike. But he added that Japan needed to strengthen its defenses generally, “given that the security situation surrounding Japan is becoming increasingly severe.”

Professor Takesada said that Mr. Abe would have to tread carefully on the issue. Opposition to his goal of undoing the Constitution’s restrictions on the military has contributed to a recent slide in his approval ratings. And extending Japan’s military reach could antagonize not only North Korea but also the South, where distrust of Japan, the Korean Peninsula’s former colonial occupier, remains entrenched.

Despite such risks, Professor Takesada said, Japan should acquire the capacity for pre-emptive strikes, if only for its potential deterrent effect. An unsettled North Korea policy in Washington under President Trump, he added, makes maximizing Japan’s own capacities more urgent.

“Short of getting nuclear weapons, which very few Japanese support, this is the best conventional way to make Kim Jong-un think twice about attacking,” he said.

Jonathan Soble reported from Tokyo, and Choe Sang-Hun from Seoul, South Korea.

A version of this article appears in print on , on Page A7 of the New York edition with the headline: Wary Neighbors Weigh Deadlier Weapons to Counter Threat From Pyongyang. Order Reprints | Today’s Paper | Subscribe